For many years infusion packets, such as tea bags were available primarily as square or round two-ply sheets of porous filter material, typically made of paper, with the infusible material, such as tea, sandwiched between the sheets. Such packets restrict the flow of infusible material within the packet substantially to two dimensions. As a result the infusion performance of such packets is limited.
Thus the past few decades have seen the development of mass-produced infusion packets which have a more three-dimensional shape and which allow the infusible substance more room to move. Of particular success have been the tetrahedral-shaped packets such as those described in the international patent applications published as WO 95/01907 (Unilever) and WO 2004/033303 (I. M. A. SPA).
In the manufacture of tetrahedral packets, the tetrahedral shape is conventionally formed by making mutually perpendicular transverse seals in a tube of filter material and apparatus designed for such manufacture is ill-suited to the manufacture of other three-dimensional shapes.
Therefore, it would be desirable to develop a process which can manufacture a variety of three-dimensional shapes, particularly shapes comprising sharp features, typified by a vertex, e.g. as provided by a tetrahedral shape.